Molecules controlled by the human HLA-D region function in immune responsiveness and have been clinically important in graft rejection and disease susceptibility. The goal of this project is to examine the genetic complexity of this region. This study will focus on the immunochemistry of the SB antigens. These antigens, controlled by a locus linked to DR, closely resemble the DR antigens in molecular structure and expression. The SB antigens will be isolated and compared to the DR antigens using amino acid sequencing and peptide mapping. These studies should elucidate the number of loci controlling the expression of the two complexes and the relationship between the SB and DR loci. SB antigen polymorphism will be examined using similar procedures. An understanding of the molecular biology of these antigens will be important in interpreting serological and functional studies of the HLA-D region. This region has been implicated in a wide variety of inheritable devastating diseases and characterization of antigens such a SB will aid in the functional dissection of this complex region.